Coupling of magnetic order to planar Bi electrons in the anisotropic Dirac metals AMnBi2 (A =Sr,Ca)

We report powder and single-crystal neutron diffraction measurements of the magnetic order in AMnBi₂ (A =Sr and Ca), two layered manganese pnictides with anisotropic Dirac fermions on a Bi square net. Both materials are found to order at T_N≈300 K in k =0 antiferromagnetic structures, with ordered Mn moments at T =10 K of approximately 3.8 _{μB aligned along the c axis. The magnetic structures are Néel type within the Mn-Bi layers, but the interlayer ordering is different, being antiferromagnetic in SrMnBi2} and ferromagnetic in CaMnBi₂. This allows a mean-field coupling of the magnetic order to Bi electrons in CaMnBi₂ but not in SrMnBi₂. We find clear evidence that magnetic order influences electrical transport. First-principles calculations explain the experimental observations and suggest that the mechanism for different interlayer ordering in the two compounds is the competition between the antiferromagnetic superexchange and ferromagnetic double exchange carried by itinerant Bi electrons.